Regeneron Pharmaceuticals, Inc. (NASDAQ:REGN) stock has been a huge winner in the past. A $10,000 investment in the biotech 10 years ago would be worth more than $200,000 today. Regeneron's shareholders have been on a roller-coaster ride, though, with the stock plunging in 2016 but bouncing back somewhat this year.

What Regeneron has done in the past isn't nearly as important to investors as what the biotech could do in the future. Is the stock now a buy? Here are the arguments for and against Regeneron.

Buy diagram with pen and calculator

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The case for Regeneron

Probably the strongest reason to buy Regeneron stock now is the same reason it made sense to buy the stock several years ago -- prospects for Eylea. Regeneron and Bayer (NASDAQOTH:BAYRY) co-market the drug, which is approved for treating wet-stage age-related macular degeneration (AMD) and diabetic retinopathy in patients with diabetic macular edema. In the first half of 2017, Eylea generated over $2.1 billion in revenue for Regeneron. Sales for the blockbuster drug continue to grow by double-digit percentages.

Regeneron is hoping to extend Eylea's reach in a couple of ways. The drug is being evaluated in a late-stage study targeting an additional indication, diabetic retinopathy, in patients who don't have diabetic macular edema. Regeneron and Bayer are also testing a combination of angiopoietin2 (Ang2) antibody nesvacumab and Eylea in mid-stage studies for treating eye diseases.

Although Eylea continues to be Regeneron's biggest moneymaker by far, the company has great expectations for Dupixent. Regeneron and partner Sanofi (NYSE:SNY) won FDA approval for Dupixent in treating eczema in March 2017. Late-stage studies are also underway for Dupixent in treating asthma. Analysts project the drug could reach annual sales of $4.7 billion by 2021.

Regeneron claims two other already-approved drugs that could become blockbusters. Kevzara, which Regeneron developed with Sanofi, gained approval for treating rheumatoid arthritis in May. Then there's PCSK9 cholesterol drug Praluent, which won FDA approval in 2015. 

On top of Regeneron's current products, the company has two new late-stage pipeline candidates. A phase 3 study of fasinumab in patients with pain due to osteoarthritis of the knee or hip began in the second quarter of this year. Around the same time, Regeneron and Sanofi initiated a late-stage study of PD-1 antibody REGN2810 as a first-line treatment for non-small-cell lung cancer. 

The case against Regeneron

The problem for Regeneron is that there's more to the story for pretty much all of its products and pipeline candidates. Eylea is going strong now but could face a formidable rival relatively soon from Novartis' (NYSE:NVS) RTH258. In two head-to-head studies, Novartis found that RTH258 was just as effective as Eylea. More importantly, the majority of patients only have to take RTH258 once every three months, instead of once every one or two months for Eylea. Novartis expects to submit its drug for regulatory approval next year.

And while Dupixent is likely to become a major success for Regeneron, the drug might not hit the most optimistic sales projections. AbbVie (NYSE:ABBV) reported positive results from a phase 2 study of JAK inhibitor upadicitinib in treating eczema. While Dupixent requires an injection, upadicitinib is a pill -- which could give it a key advantage if it ultimately wins approval.

Kevzara is promising, but the drug faces a very crowded market. Praluent still hasn't picked up significant sales momentum, as payers still appear to be reluctant to open up the purse strings for the high-priced cholesterol drug. In addition, Regeneron and Sanofi face Amgen in court over alleged patent infringement.

Turning to the pipeline, safety concerns resulted in the FDA's putting fasinumab on clinical hold last year. Although Regeneron and partner Teva structured the late-stage study for the drug in a way that attempted to address the concerns, there's no guarantee the drug will be successful. As for REGN2810, Regeneron and Sanofi will be late to the party, with two PD-1 inhibitors already on the market.

Regeneron stock trades at 26 times expected earnings. That wouldn't seem too steep if the biotech had clear skies ahead of it for its products and pipeline. But it doesn't.

Is the stock a buy?

Despite the prospects of competition for Eylea, Dupixent, and Regeneron's other drugs, I expect the stock to climb in the next few years. I think Eylea will continue to be a big winner for Regeneron and also project that Dupixent will enjoy strong sales growth.

However, just because a stock is likely to go up doesn't make it one to buy. In my view, there are several other biotech stocks that are in better position for growth (AbbVie, for example). Regeneron isn't a bad stock, but I think investors can get better returns elsewhere.

Keith Speights owns shares of AbbVie. The Motley Fool has no position in any of the stocks mentioned. The Motley Fool has a disclosure policy.